Synthesis, structure, and interconversion of chiral rhenium oxygen- and sulfur-bound sulfoxide complexes of formula [(η5-C5H5)Re(NO)(PPh3)(OS(Me)R)]+ X−; diastereoselective oxidations of coordinated methyl alkyl sulfides

Abstract

Reactions of the chiral chlorobenzene complex [(η⁵-C₅H₅)Re(NO)(PPh₃)(ClC₆H₅)]⁺BF₄⁻ (1⁺ BF₄⁻) and alkyl methyl sulfoxides OS(Me)R (R = a, Me; b, Et; c, i-Pr or d, t-Bu), at −15 °C gave the oxygen-bound sulfoxide complexes [(η⁵-C₅H₅)Re(NO)(PPh₃)(OS(Me)R)]⁺ BF₄⁻ (2a–2d⁺BF₄⁻; 94–56%). Above 0 °C, 2a–2d⁺BF₄⁻ rearrange to sulfur-bound linkage isomers [(η⁵-C₅H₅)Re(NO)(PPh₃)(S(O)(Me)R)]⁺BF₄⁻ (3a–3d⁺BF₄⁻; 96–20%). The triflate salts 2c,2d⁺TfO⁻ and 3b,3c⁺TfO⁻ are analogously prepared from (η⁵-C₅H₅)Re(NO)(PPh₃)(OTf). Complexes 2b–2d⁺X⁻ and 3b–3d⁺X⁻ can exist as two Re, S configurational diastereomers. Stereochemistry is assigned from reactions of (S)- or (R)-1⁺BF₄⁻ with enantiomerically enriched sulfoxides (R)-b–d, and a crystal structure of (S_ReR_S,R_ReS_S)-2d⁺TfO⁻·0.5CH₂Cl₂. Relative diastereomer stabilities, and the basis for the divergent kinetic and thermodynamic oxygen/sulfur binding selectivities, are analysed. The alkyl methyl sulfide complexes [(η⁵-C₅H₅)Re(NO)(PPh₃)(S(Me)R)]⁺ X⁻ and dimethyldioxirane react (acetone, −20 °C) to give 3a–3d⁺X⁻. Diastereoselectivities (3b–3d⁺X⁻) are fair to good. However, some overoxidation to Ph₃PO occurs, lowering yields.